(1) Field of the Invention
This invention relates to an ion-sensitive FET sensor and, more particularly, to an ion-sensitive FET (hereafter referred to as an "ISFET") sensor for measuring the ionic concentration of a solution by the potentiometric response of an electrode. The ISFET sensor disclosed herein is especially suited for measurement of ionic concentration within a living body.
(2) Description of the Prior Art
A variety of chemical semiconductor instruments comprising a chemical sensor and a semiconductor device in integrated form have been developed in recent years. One example is the aforementioned ISFET, which is an integrated combination of of a MOS-type field-effect transistor (MOSFET) and an ion-sensitive electrode. The ISFET is a sensor of a novel type radically different from the ion sensors of old and can be made very small in size, exhibits such excellent characteristics as the high input impedence and low noise of the MOSFET, and has a very short response time. Owing to these outstanding sensor features, the ISFET has attracted much attention.
However, the ISFETs investigated so far basically have been adapted for use as pH sensors, and only recently have sensors for measurement of ion concentrations other than hydrogen ion begun to be studied. The structure of an ion-sensitive layer is of prime importance for obtaining an ISFET having excellent sensor characteristics. Problems are encountered in terms of the adherability and water-resistant property of the ion-sensitive layer, and all ion sensors generally share the significant drawback of potential drift. Potential is unstable, characteristics deteriorate due to the adsorption of proteins and the like, and clotting tends to occur when measurement is made in a blood sample.
It is known from, e.g. the specification of Japanese Patent Application Laid-Open No. 59-164952, that potential drift can be reduced by adopting a structure in which a polymeric membrane layer devoid of an ion-sensitive substance is interposed between a gate isolating layer and an ion-sensitive layer for the purpose of improving the adhesion between these two layers. However, as a result of the poor electronic conductivity of the intervening polymeric membrane layer, an ion sensor having such a structure exhibits a high membrane impedence and is susceptible to noise and other disturbances when a measurement is made.
It is known from, e.g. the specifications of Japanese Patent Application Laid-Open Nos. 57-63444, 60-73351, that a highly durable ion sensor having an ion-sensitive layer of improved adherability is obtained by adopting a structure in which a metallic membrane is interposed between the gate isolating layer and the ion-sensitive layer. However, the problem with such an expedient is poor stability ascribed to the fact that the gate portion of the sensor is readily influenced by oxygen.
Accordingly, there is demand for an ion sensor having good adhesion between the ion-sensitive layer and the gate portion, outstanding durability and excellent stability with little susceptibility to the effects of interfering ions.
As for ion sensors for measurement of pH, the few known use SiO.sub.2, SiN.sub.4, Al.sub.2 O.sub.3 or Ta.sub.2 O.sub.5 as a hydrogen ion-sensitive layer. Moreover, the ion sensor that relies upon Si.sub.2 O.sub.3 as the hydrogen ion-sensitive layer generally exhibits unstable operation and has little utility value. Ideal hydrogen ion-sensitive layers for pH measurement ISFETs exhibiting good pH response and little interference from other ions such as alkali metal ions are limited to membranes consisting of chemically stable compounds such as Al.sub.2 O.sub.3 and Ta.sub.2 O.sub.5.